Field of the Invention
The invention relates to a magnetic-inductive flowmeter having at least one measuring tube for the flow of an electrically conductive medium, having at least one magnetic field generator for generating an alternating magnetic field running at least also perpendicular to the longitudinal axis of the measuring tube, having at least two measuring electrodes—in particular being in contact with the medium—and having an evaluating circuit, wherein the magnetic field generator has at least one field coil, preferably one current regulator, preferably one switching bridge and preferably one microcontroller. The invention also relates to a method for operating a magnetic-inductive flowmeter.
Description of Related Art
A magnetic-inductive flowmeter of the above-described type is known from DE 199 07 864 A1 and corresponding U.S. Pat. No. 6,453,754. The magnetic field generator can have a field coil or two field coils in this known magnetic-inductive flowmeter. This is the reason that it is described above that the magnetic field generator has at least one field coil. The magnetic field generator of the known magnetic-inductive flowmeter also has a current regulator, a switching bridge and a microcontroller. However, because neither a current regulator, nor a switching bridge, nor a microcontroller is functionally required, it is described above that the magnetic field generator has preferably one current regulator, preferably one switching bridge and preferably one microcontroller.
Magnetic-inductive flowmeters have been known extensively for decades. Reference is thus made, as an example, to the citation “Technische Durchflussmessung” by Prof. Dr.-Ing. K. W. Bonfig, 3rd edition, Vulkan-Verlag Essen, 2002, pages 123 to 167 and also to the citation “Grundlagen Magnetisch-Induktive Durchflussmessung” by Dipl.-Ing. Friedrich Hoffmann, 3rd edition, 2003, publication of the company KROHNE Messtechnik GmbH & Co. KG.
The basic principle of a magnetic-inductive flowmeter for flow measurement of a flowing medium goes back to Michael Faraday, who proposed, in 1832, the use of the principle of electromagnetic induction for measuring the flow velocity of an electrically conductive medium.
According to Faraday's law of induction, electric field strength is formed perpendicular to the direction of flow of the medium and perpendicular to the magnetic field in a flowing, electrically conductive medium interfused by a magnetic field. Faraday's law of induction is thus exploited in magnetic-inductive flowmeters in that a magnetic field fluctuating over time during the measurement process is usually generated by means of a magnetic field generator usually having at least one magnetic field coil, and that the magnetic field at least partially interfuses the electrically conductive medium flowing through the measuring tube. Here, the generated magnetic field has at least one component perpendicular to the longitudinal axis of the measuring tube or perpendicular to the direction of flow of the medium.
If it is mentioned above that the magnetic-inductive flowmeter being discussed here has at least one magnetic field generator “for generating a magnetic field running perpendicular to the longitudinal axis of the measuring tube”, then it should also be mentioned here that the magnetic field preferably runs perpendicular to the longitudinal axis of the measuring tube or perpendicular to the direction of flow of the medium, however, it is sufficient when a component of the magnetic field runs perpendicular to the longitudinal axis of the measuring tube or perpendicular to the direction of flow of the medium.
It is also described above that the magnetic-inductive flowmeter being discussed here also has at least two measuring electrodes for tapping a measuring voltage induced in a flowing medium, wherein the measuring electrodes preferably come into contact with the medium. Preferably, the virtual connection line of the two measuring electrodes runs at least essentially perpendicular to the direction of the magnetic field interfusing the measuring tube perpendicular to the longitudinal axis of the measuring tube. In particular, the measuring electrodes can be provided in such a manner that their virtual connection line actually runs—more or less—perpendicular to the direction of the magnetic field interfusing the measuring tube.
Finally, it is described above that the measuring electrodes are, in particular, such that they come into contact with the medium. Indeed, of course, the electric field strength generated by induction in the flowing, electrically conductive medium can be tapped by direct, i.e., galvanic, measuring electrodes in contact with the medium as a measuring voltage. However, there are magnetic-inductive flowmeters in which the measuring voltage is not tapped by direct, i.e., non-galvanic, measuring electrodes in contact with the medium, rather the measuring voltage is capacitively determined.
Magnetic-inductive flowmeters in the industrial sector were initially operated using an alternating magnetic field. For financial reasons, the field coil or field coils is/are connected to the existing AC voltage supply, so that the magnetic field changes its field strength essentially sinusoidally. The operating of magnetic-inductive flowmeters with a magnetic field that changes its field strength essentially sinusoidally, however, has disadvantages (see DE 199 07 864 A1, column 1, line 53 to column 2, line 13, and paragraph spanning columns 1 & 2 of corresponding U.S. Pat. No. 6,453,754).
Since the middle of the 70s, magnetic-inductive flowmeters operating with a switched constant magnetic field have established themselves, in which switched direct current is used as coil current. If a switched constant magnetic field is used, disadvantages are avoided that occur when a magnetic field is used, however, there are also problems involved with using a switched constant magnetic field (compare DE 199 07 864 A1, column 2, lines 18 to 41 and second full paragraph of corresponding U.S. Pat. No. 6,453,754).
The invention that is described in DE 199 07 864 A1 and corresponding U.S. Pat. No. 6,453,754 has the object of designing and further developing the known magnetic-inductive flowmeter operating with a switched constant magnetic field in that the described, system-related switchover phases are shorter than in the magnetic-inductive flowmeters previously known from the prior art, so that the field frequency, i.e., the frequency at which the constant magnetic field is switched, can be increased (compare DE 199 07 864 A1, column 2, lines 42 to 49 and first paragraph of the Summary of the Invention of corresponding U.S. Pat. No. 6,453,754).
In detail, an additional current source is provided in the known magnetic-inductive flowmeter and, by means of the additional current source, an additional current can be fed into the field coil or into the field coils immediately at the beginning of each half cycle of the coil current, presently switched constant current (see, DE 199 07 864 A1, column 2, lines 50 to 57, and second paragraph of the Summary of the Invention of corresponding U.S. Pat. No. 6,453,754).
Of course, there are magnetic-inductive flowmeters of very different nominal sizes and having very different constructional geometries. This leads to the electrical characteristics of the field coil or field coils being very different from one another, in particular the inductance and the resistance of the field coils used for different magnetic-inductive flowmeters. Thus, the time constants of the field coil dependent on the ratio of inductance to resistance are very different.
If one does not want to design a coil current supply for each magnetic-inductive flowmeter having a different nominal size, then a coil current supply that can be used for the very different field coils should be designed so that, for example, the desired current can flow, even at a relatively large resistance, and so that, at a relatively large inductance and a relatively small resistance, the settling time dependent on the time constants is not too large. Thus, the object of the invention is to provide a magnetic-inductive flowmeter in which a certain coil supply current for otherwise very different magnetic-inductive flowmeters can be used, namely, in particular, for magnetic-inductive flowmeters having very different field coils and/or with very different demands on the required field strength.